1. Field of the Invention
The present invention relates to a method of and apparatus for measuring a very low water content in non-condensible gases which has a dew point of -80.degree. C. or less.
2. Description of the Related Art
As a result of the rapid developments in technology, utilization of inert gases, for example, N.sub.2, Ar, H.sub.2 and He, which have very low water contents, i.e., 0.5 ppm or less, (dew points of -80.degree. C. or less) has increased. In particular, material and carrier gases which are used for epitaxial growth and CVD in the semiconductor industry are required to have ultrahigh purities, and there is therefore a need to measure dew points of -80.degree. C. or less with a high degree of precision.
Many means for measuring the water content of a gas have heretofore been known. For example, as a means for measuring a dew point, one method has been known for a long time in which a change in the intensity of light which occurs when moisture contained in the gas to be measured has condensed on a cooled mirror surface is optically detected. As one type of optical detection method, absolute measurement is described in, for example, Industrial Instrumentation Handbook (Asakura Shoten, 1976), p. 297. According to this method, a variation in the intensity of the light reflected from a mirror surface is detected by the use of a freezing medium, a refrigerating machine and a thermoelectric cooler, and the temperature of the mirror surface at the time when any change in the light intensity is detected is measured to thereby obtain a humidity value which is basically taken from the dew point. The apparatus employed to practice this method has a cooled surface onto which light is projected from a light-emitting diode or a white light source, and the reflected light from the cooled mirror surface is received by a photodetector. In general, the mirror surface is cooled by a Peltier element, and as the surface is cooled, moisture contained in the gas to be measured becomes condensed or frozen on the cooled surface, so that the light projected on the adsorbed water molecules is partially absorbed or scattered, resulting in a decrease in the intensity of the reflected light. The change in the intensity of the reflected light is detected by the photodetector, and the temperature of the cooled mirror surface at the time when the intensity change occurs is measured to thereby measure the dew point of the gas concerned. There have been a rather large number of studies of dew-point instruments designed to measure the water content of a gas for industrial purposes with even more precision and on a continuous basis by using changes in the intensity of reflected light, e.g., Japanese Patent Publication (Kokai) Nos. 58-113840 (1983), 61-75235 (1986), U.S. Pat. No. 4,335,597, U.S. Pat. No. 4,345,455, etc.
Japanese Patent Publication (Kokai) No. 58-113839 (1983) also shows a method of measuring a dew point by detecting a change in the intensity of light scattered at an angle different from the reflection angle, rather than by measuring a change in the intensity of reflected light. More specifically, when moisture becomes condensed or frozen on a mirror surface, a sudden change occurs in the intensity of the light scattered by the condensate (or ice). Therefore, the temperature of the cooled mirror surface at the time when such an intensity change occurs is measured to thereby measure the dew point of the gas concerned. The principle of this method has been known for a long time and is described in Humidity and Moisture (Vol. 1), p. 165, Reinhold Publ. Co., New York, 1965. However, the measurable range of this method is shown to be between -73.degree. C. and 49.degree. C., and it is therefore impossible with this prior art to realize measurement of low temperature dew points, as in the case of the present invention. In addition to the methods wherein the amount of water in a gas with a very low water content is measured by detecting an optical change that occurs when moisture contained in the gas becomes frozen on a cooled mirror surface, other methods have also been put into practical use wherein the amount of water in a gas with a very low water content is measured by detecting a change in the frequency of a crystal oscillator or a change in the electrostatic capacity which is caused by adsorption of water. However, none of these conventional methods is capable of measuring a dew point of -80.degree. C. or less with high accuracy.
The above-described method wherein the dew point of a gas is measured by detecting a decremental change in the intensity of the reflected light at the time when moisture contained in the gas has become condensed or frozen on a cooled mirror surface suffers from the problem that, as the water content in the gas becomes lower, that is, as the dew point becomes lower, the amount of condensate (or ice) decreases to an extreme degree, so that it is difficult to measure the dew point with a high degree of precision. Further, the prior art has the disadvantage that, when a gas flows slowly on a cooled mirror surface in the conventional arrangement, condensation (or freezing) of an extremely small amount of moisture takes place very slowly, and therefore no distinct change in the reflected light is shown, particularly when the dew point to be measured is -80.degree. C. or less. In the case of a very low water content, i.e., 10 [ppb] or less, water molecules are adsorbed on the mirror surface very gradually, so that it has heretofore been difficult to read any sudden change in the intensity of the reflected light. It is therefore effective to blow the gas to be measured against a cooled mirror surface from a nozzle-shaped (or pipe-shaped) member. However, with respect to a gas having a dew point of -80.degree. C., which has heretofore been considered capable of measurement with a substantially good degree of accuracy, it is necessary to precisely read the temperature of the cooled mirror surface at the time when the moisture contained in the gas in an extremely low amount, i.e., about 1 ppm, becomes frozen on the mirror surface; for a dew point of -110.degree. C., this amount is about 1/1000 of 1 ppm; and for a dew point of -136.degree. C., 1/1000 of 1/1000 of 1 ppm, that is, 1 ppt. There has accordingly been a demand for a technique of precisely measuring such a freezing point.